1. Field of the Invention
The invention is generally related to the field of the production of an organic photovoltaic device (OPVd), a solar cell based on conjugated semiconducting polymers, and more specifically to the field of electrochemical methods for the deposition of hole extraction layers and the usage of the layers in the construction of solar cells.
2. Prior Art
Solar cells convert the energy of sunlight directly into electricity by the photovoltaic effect. Solar cells are made from semiconductor materials that can be either inorganic or organic compounds. The first generation of solar cells is based on crystalline silicon. Silicon is doped by introducing other chemical elements to form n- and p-conducting layers. As a result of light absorption charge separation happens on the interphase. Electrons travel through the n-conducting layer and positively charged holes through the p-conducting layer creating current in an external circuit. Crystalline silicon is produced by energy and time consuming melting techniques such as the Czochralski method, which is the main reason for the high price of crystalline silicon solar cells.
The second generation of solar cells is thin-film solar cells. The semiconducting materials used to fabricate thin-film solar cells are low-cost amorphous silicon, cadmium telluride, copper indium gallium diselenide, and other inorganic semiconductors. Thin photoactive layers are deposited either by vapor deposition or printing methods. This decreases coast of manufacturing from about US$5.00 per watt down to about US$2.00 per watt, and also brings lightness and some flexibility. However, the efficiency drops greatly, especially for amorphous silicon, and usually does not exceed 10%.
The third generation of solar cells is based on organic semiconducting materials. Organic semiconductors, both low molecular weight compounds and conjugated polymers, are used to make extremely thin and even semitransparent solar cells with the thickness of photoactive layers as low as 300 nm. Organic photovoltaic devices (OPVd) are by far lighter, more flexible and cheaper than any other solar cells. Hole extraction and photoactive layers in OPVd are currently deposited either by solution processing techniques, like spin-coating and doctor blade, or by inkjet printing.
Poly(3,4-ethylenedioxythiophene) (PEDOT) has been studied intensively for the past two decades due to high conductivity and excellent stability. PEDOT has found many applications in antistatic coatings, sensors, organic light-emitting diodes, electrochromic windows, etc. In the doped state PEDOT has good transparency in the UV-vis region, helping its adoption as the most widely used hole-extracting material in organic photovoltaics (OPV).
PEDOT was first synthesized by scientists from Bayer AG Laboratories in the late 1980s. Chemical polymerization of 3,4-ethylenedioxythiophene (EDOT) was done by Jonas et al., and the commercial name Baytron was given to PEDOT. PEDOT layers showed exceptional stability and transparency in doped oxidized state as well as high conductivity up to 300 S/cm. In 1991, the solubility problem was overcome by polymerizing EDOT in the presence of polystyrenesulfonate (PSS). Polymerization was done in an aqueous solution resulting in the water-soluble PEDOT:PSS complex (Baytron P, where P stands for polymer). This stimulated tremendous research activity and a wide variety of PEDOT applications in organic electronics.
The electrochemical polymerization of PEDOT was first performed by Dietrich and Heinze4 and has been attracting research interest as an alternative method to solution casting techniques. The mesomeric effect (+M) of oxygen atoms stabilizes cations during polymerization and thus decreases oxidative potential down to 1.25 V vs SHE from 1.95 V for unsubstituted thiophene. This allows electrochemical deposition from aqueous solution. EDOT itself has low solubility in water but can be solubilized and polymerized in the presence of amphiphilic molecules such as sodium dodecyl sulfate, PSS, dodecylbenzenesulfonic acid, cyclodextrin, and others. However, in this case an electrodeposited layer is contaminated with surfactants reducing conductivity and limiting applications where high hole mobility is desired. Polymerization in organic solvents eliminates the problem. Polymerization is typically done in acetonitrile using LiClO4 as electrolyte resulting in sky-blue doped PEDOT layer containing ClO4− as counterions. Xia et al. deposited thin layers of PEDOT and performed simultaneous electrochemical surface plasmon spectroscopy. Dielectric constant and thickness of the layers could be measured accurately by this technique as well as electrochromic switching between different redox states. Nucleation stage and early growth was investigated by Randriamahazaka et al. The combined mechanism of progressive diffusion-controlled 3D nucleation and instantaneous 3D charge transfer as a limiting factor was proposed. Su-Moon Park et al. conducted morphological studies of growing PEDOT films. By using current-sensing atomic force microscopy, they were able to measure conductivities of the layers at the same time. It was shown that the current value fluctuates drastically from one point to another; morphology and electrical properties of the film were found to be sensitive to deposition conditions. Later, the electrochemical characteristics of electrodeposited PEDOT layers were found to be much better compared to spin-coated PEDOT-PSS. It can be concluded that electrochemistry is a powerful technique to control thickness, morphology, and redox state of the in situ deposited PEDOT layer. Despite these detailed investigations of electrochemically deposited PEDOT films, their application in organic electronics and particularly as a hole extraction layer in OPV is narrow. Electrodeposited PEDOT was successfully incorporated into ZnS/ZnO dye-synthesized and hybrid ZnO/poly[2-methoxy-5-(30,70-dimethyloctyloxy)-1,4-phenylenevinylene] (ZnO/MDVO-PPV) solar cells. Kuo-Chuan Ho et al. electrochemically polymerized EDOT and its derivatives on ITO from a boron tri$uoride-ethyl ether solution. Porous layers were obtained and porosity was observed to be higher for derivatives with bulkier groups as a result of π-stacking distortion. An active layer of regioregular poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (rrP3HT-PCBM) was deposited on top by spin-coating and the prepared OPVd were characterized. Efficiency as a function of porosity was calculated and found to be 3.57% for moderate porosity.
Since the first heterojunction organic solar cell with 1% efficiency was reported by Tang in 1986, tremendous amount of research has been conducted in this field. In 1993, Sariciftci applied fullerene as an electron acceptor material in the solar cell where a semiconducting polymer was used as a donor. Enhanced photoelectron charge transfer was observed due to high electron affinity of fullerene. In addition, fullerene has good transparency and conductivity which made it the most widely used acceptor in OPVd. A bulk heterojunction concept was introduced in 1994 by Yu and Heeger and was later shown to improve exciton dissociation. However, efficiency of the organic solar cells remained low until it was realized that morphology of the photoactive layer is a key factor. Nanodomains of the donor component should be of the same size as the exciton diffusion length, which is about 10 nm for most of the organic semiconductors. For better charge transport the donor and the acceptor should form bicontinuous network. An ideal structure is an array of nanorods aligned perpendicular to the surface of electrodes. Upon the selection of proper concentration, solvent, evaporation rate and annealing technique, the morphology can be improved in and the efficiency up to 5% has been reached. Instinctively, the nanofibrillar structure of a semiconducting polymer should provide high hole mobility. Thin nanofibers of poly(3-alkylthiophenes) with diameter down to 10 nm have been successfully prepared either by precipitation from a mixed solvent or by slow cooling of dichlorobenzene solution. With the high crystallinity and the long length of the fibers, the efficiency has been shown to be on the level of 3%. The reason is the improper alignment of the nanofibers deposited from the solution. As a result of the solvent evaporation they oriented in parallel to the surface instead of preferable perpendicular direction.
Most of the semiconducting polymers can be polymerized electrochemically. In this case polymerization is coupled with the layer deposition. One important advantage is that monomers without side alkyl groups can be used since no solution processing of the polymer is needed. Potentially, by using low-cost starting materials and simple equipment setup, the electrochemical method may potentially decreases the cost of the OPVd fabrication. Electrochemistry is a powerful technique to control not only thickness of the deposited layer but also its morphology. The layer grows bottom-up, giving the possibility to obtain brush-like structures preferred for the OPV application. Besides that, it has been shown that the electrical contact between electrode and the electrochemically deposited layer is stronger than for the solution casted one. All these advantages make electrochemical method a promising alternative for the OPVd preparation. Despite that, application of the method is barely described in literature. Electrochemically polymerized PEDOT was successfully used as a hole extraction layer. Electrodeposition typically was shown to results in rough films with large surface area improving contact with a photoactive layer. For the same reason open circuit voltage (Voc) is lower than for the smooth spin coated PEDOT-PSS. Efficiency of up to 3.57% was measured for the cells where spin coated P3HT-PCBM was used as photoactive layer. Optoelectronic properties of the electrodeposited polythiophene and its derivatives were studied in single layer photovoltaic cells. In the absence of the strong electron acceptor, low efficiency of about 0.01% was measured. Ratcliff et al made two-layer OPVd by vacuum deposition of fullerene on electrochemically polymerized P3HT. C60 penetrated into thin textured P3HT layer increasing the donor-acceptor interfacial area. Even though electropolymerization leads to the regiorandom P3HT relatively high Isc of 3 mA/cm2 and 1% overall energy conversion efficiency were measured. By linking fullerene covalently to the thiophene monomer it can be codeposited electrochemically resulting in so-called double-cable polymers which have been studied intensively for over a decade. However, photoresponse of this type of polymers is limited by fast exciton recombination process. Fan et al codeposited fullerene and unsubstituted polythiophene electrochemically from 1-chloronaphthalene/BFEE solution. The technique was not effective for the fullerene deposition and the performance of the fabricated device was poor because of the low fullerene content. Balch studied electrodeposition of fullerene in detail. It was shown that fullerene molecules undergo radical polymerization at negative reductive potential in the presence of small amounts of fullerene epoxide or oxygen catalyzing the process. As a result yellowish films of polyfullerene were obtained.
Polymer solar cells have been widely investigated in the last two decades due to potentially low cost, flexibility and light weight. However, lack of the efficiency and stability limits their vast industrial expansion. Generation of photocurrent in an organic photovoltaic device (OPVd) is a complex presses consisting of photon harvesting, exciton generation and migration, charge separation and transport. Morphology has a critical effect on the performance of the polymer solar cells. Due to the limited exciton diffusion length donor and acceptor components should form a fine mixture on nanolevel. Besides that, a bicontinuous network is desired for improved charge transport. Traditional ways to fabricate such a polymer solar cell are based on solution processing techniques or printing methods.
It is well-known that conjugated polymers can be synthesized electrochemically. The electropolymerized layer is usually rough and grows upwards from the working electrode surface. So, by filling openings with an acceptor component, the bicontinuous brush-like architecture might be achieved. The polymerization and the layer deposition are combined in one process, hence there is no need for the polymer processing. For the same reason simple monomers can be used as the solubility of the polymer is not an issue. In addition, electrochemistry gives control over thickness of the deposited layer and oxidation state of the polymer.
There are number of publications where electropolymerized PEDOT was applied as a hole transporting layer to build OPVd, hybrid or dye-sensitized solar cells. Recently, we studied PEDOT layers deposited electrochemically from mixed toluene/acetonitrile solvent. The layers had fibrillar brush-like morphology. For the thin to moderate thicknesses the diameter of the fibers was as small as 20 nm resulting in the high surface area. The layers were incorporated into OPVd and 30% improvement in the fill factor compared to spin-coated PEDOT-PSS layer was observed. Electrodeposited conjugated polymers were successfully used in hybrid organic-inorganic solar cells. Gong et al developed electropolymerization of doped poly(3-methylthiophene) (P3MT) layer on CdS to form a Schottky junction and efficiency up to 4% was measured. Other examples include P3MT on CdSe, P3MT on CuInSe2, P3MT on TiO2 and polypyrrole on CuInSe2. A conjugated polymer in its neutral undoped state together with an acceptor (e.g. fullerene) are essential components of the polymer solar cell. Even with all the advantages already discussed, electrochemistry has been barely used to fabricate a polymer solar cell. One method is based on electropolymerization of fullerene-derived thiophene monomer leading to so-called double-cable polymers. Another example is evaporation of fullerene on top of the electropolymerized textured poly(3-hexylthiophene) (P3HT) film. Despite the regiorandom structure of P3HT relatively high Jsc of 3 mA/cm2 and efficiency of 1% were measured. Fan et al performed electrochemical co-deposition of polythiophene and fullerene from thiophene/fullerene solution. However, polythiophene had poor conjugation length and amount of deposited fullerene was insignificant. Balch et al developed an effective way for the electrochemical deposition of fullerene. Addition of small amounts of fullerene epoxide was shown to promote electrochemical reductive polymerization of fullerene resulting in uniform layer on a working electrode surface.